1. Field of the Invention
The invention relates to an arrangement having a coated piezoelectric actuator with at least one piezoelectric element.
2. Description of the Prior Art
It is known per se that for constructing the aforementioned piezoelectric actuator, a piezoelectric element can be used in such a way that by utilizing what is know as the piezoelectric effect the needle stroke of a valve or the like can be controlled. The piezoelectric element is constructed of a material with a suitable crystalline structure such that upon application of an external electrical voltage, a mechanical reaction of the piezoelectric element ensues, which depending on the crystalline structure and the regions where the electrical voltage is applied represents compression or tension in a predeterminable direction. Such piezoelectric actuators are suitable for instance for applications in which reciprocating motions take place under strong actuation forces and at high pulse frequencies.
For instance, one such piezoelectric actuator is known as a component of a piezoelectric injector from German Patent Disclosure DE 10026005 A1, which can be used to trigger the nozzle needle in injectors for injecting fuel into the combustion chamber of an internal combustion engine. In this piezoelectric actuator, a piezoelectric element is constructed as a stack of a plurality of electrically coupled-together piezoceramic layers, and this stack is held between two stops by prestressing, via an actuator foot and an actuator head. Each piezoceramic layer is fixed between two inner electrodes, by way of which an electrical voltage can be applied from outside. Because of this electrical voltage, the piezoceramic layers then each execute short reciprocating motions in the direction of the potential drop, and these motions add together to make the total stroke of the piezoelectric actuator. This total stroke is variable by way of the level of the voltage applied and can be transmitted to a mechanical final control element.
Such known arrangements are often used as so-called common rail systems for injecting fuel into direct-injection diesel engines. In these systems known as common rail injectors, the injection pressure can be adapted easily to the load and rpm of the engine.
These common rail injectors may be constructed such that there is a nozzle needle controlled directly by the piezoelectric actuator, and the piezoelectric actuator is surrounded directly or indirectly by the pressure of the fuel, and between the nozzle needle and the piezoelectric actuator, a hydraulic coupling chamber is provided. It is important here that the relatively vulnerable piezoelectric actuator, in the internal volume of a holder body that is filled with fuel at high pressure, be neither attacked nor destroyed by that fuel.
Various methods for sealing off the piezoelectric actuator, especially if it is located in the low-pressure region of a piezoelectric injector, from the surrounding medium are known per se. For instance, such a piezoelectric actuator can be encapsulated in a stable metal sheath, and the actuator head, which transmits the stroke and the force of the piezoelectric actuator to the outside, is located on the mechanically active end face of the piezoelectric actuator. This actuator head is then connected to the stable sheath via a diaphragm. As a result, the diaphragm creates a flexible, tight connection between the actuator head and the sheath.
In an application of the piezoelectric actuator in the high-pressure range of a piezoelectric injector, however, the aforementioned embodiment is disadvantageous with regard to the strength of the diaphragm as well as the strength of the sheath. From German Patent disclosure DE 10217361 A1, an embodiment is known in which a single piezoelectric actuator is first coated on portions of its surface with an organic base layer, which in turn is provided with an inorganic cover layer that has the task of sealing off the base layer from the surrounding fuel that would otherwise attack the organic base layer.
It is furthermore known, for instance from International Patent Disclosure WO 0261856 A1, that in connection with a piezoelectric actuator, a shrink-fit hose is first slipped over the piezoelectric element and the actuator foot and the actuator head. Next, the interstice between the shrink-fit hose and the piezoelectric actuator is filled with a filler material, and finally, the shrink-fit hose is shrunk by thermal action and as a result the piezoelectric actuator and the filler material are tightly enclosed.
As a rule, these known arrangements can be realized only with very great effort and high expense.